What drives conspiracy theorists?

How is it that some people become completely caught up in believing in (patently) ridiculous conspiracies? To the exclusion of logical thought, evidence, common sense, and public knowledge?

It’s not just those who believe in aliens and bigfoot and a faked lunar landing. Senator Jim Inhofe believes that climate change is one of the major hoaxes ever perpetrated on the American people. He blames Al Gore, the United Nations, and the Hollywood elite. “I would add to that list MoveOn.org, George Soros, Michael Moore, and a few others,” he said. (borrowed from Sen.Bernie Sanders)

Is it a mental illness? A form of manipulation and attention seeking? Intellectual incapacity?

What is creating/promoting/supporting this bizarre subculture?